1. Field of the Invention
The invention relates to a pebble bed reactor comprising a core cavity which is filled with spherical fuel elements and bounded by reflector material, and through which spherical fuel elements pass under the effect of gravity and coolant gas flows in a descending or ascending stream.
2. Background Information
Pebble bed reactors are known. For example, a pebble bed reactor with graphite core is described in DE 31 49 794 C1. In pebble bed reactors, a pile of spherical fuel elements known as the pebble bed passes under the effect of gravity through a core cavity bounded by reflector material. Control of the nuclear reactor is achieved by neutron absorbers, for example control rods, which either are moved in bores in the reflector material or are inserted directly into the pile of spherical fuel elements for reactor control.
To replace spent spherical fuel elements, which are removed from the core cavity, the pile is refilled. Pebble outlet tubes are usually provided at the bottom of the core cavity for removal of spherical fuel elements. The fuel elements are fed toward the pebble outlet tubes via funnel-shaped depressions in the bottom reflector. Larger core cavities or core cavities with a central column of graphite in the pebble bed are equipped with a plurality of pebble outlet tubes.
Usually a stream of coolant gas, especially helium, is passed vertically through the pile of fuel elements. The direction of flow of the coolant gas can be the same as that of the direction of movement of the spherical fuel elements in the gravity direction (descending flow), but the coolant gas can also flow through the core cavity counter to the direction of movement of the spherical fuel elements, or in other words upward (ascending flow). For supply and removal of coolant gas, the core cavity is provided in the top and bottom reflectors with coolant-gas inlet and outlet orifices, which usually discharge into respective gas plenums, which are used to make the gas stream uniform, and especially to equalize the temperature in the coolant gas.
As the spherical fuel elements move through in gravity direction, the spherical surfaces become abraded. It is even possible for fragments of spheres to be formed and to accumulate at the bottom of the core cavity. Fragments of spheres are formed in particular when control rods are inserted into the pile and moved in the pile. Such fragments of spheres are mainly removed from the core cavity via the pebble outlet tubes, but they can also stick in the gas penetrations in the bottom reflector and cause fouling. In such cases, the coolant-gas flow can be greatly impaired.